Prague’s main square in the old town would have to be one of the most beautiful squares in Europe. I love the varied architecture and the fun atmosphere.
I’m always drawn to Tyn Church which dominates the square. Every building is of interest though with most of them being pure works of art. The astronomical clock is a bit overrated, I think, but plenty of tourists hang around on the hour to check it out, just watch out for pickpockets if you choose to watch.
From the main square you can easily get to the Jewish quarter and Charles Bridge.
Prague is incredibly crowded in the summer months so I recommend visiting off season if possible.